Biaxially oriented polyester film for a membrane switch

ABSTRACT

A biaxially oriented polyester film for a membrane switch, which is made from a polyester containing at least 80 mol % of ethylene-2,6-naphthalene dicarboxylate recurring units and which has an endothermic peak with an endothermic energy of at least 0.4 mJ/mg at a temperature of 110 to 160° C. 
     This film is produced by subjecting a biaxially oriented polyester film produced from the above polyester in accordance with a method known per se to the following steps in the order named: 
     (1) the step of heating the film under no strain at a temperature of 150 to 180° C. for 1 to 5 hours; and 
     (2) the step of heating the film in an unrolled or rolled state at a temperature of 80 to 122° C. for 5 to 200 hours.

TECHNICAL FIELD

The present invention relates to a film for a membrane switch and, morespecifically, to a biaxially oriented polyester film for a membraneswitch, which is made from a polyester comprisingethylene-2,6-naphthalene dicarboxylate as a main recurring unit andwhich has excellent deformation resistance during its use at hightemperatures.

BACKGROUND ART

As disclosed at page 111 of the "'85 edition of Switch LatestTechnology" published by Sogo Gijutsu Syuppansha on Sep. 28, 1984, amembrane switch comprises a spacer between two base films to define aspace between the two base films and contact points (electrodes)arranged on the opposed surfaces of the two base films which face eachother with the space in between. By depressing one of the base films,two contact points are brought into contact with each other to conductelectricity and by releasing the depressed base film, they aredisconnected from each other, thus effecting a switching function.

In recent years, such a membrane switch has been frequently used as akey board switch for calculators and personal computers, a panel switchfor the remote controller of TVs and VTRs, and the like.

Since this membrane switch effects a switching function by alternatingbeing depressed and released, flexibility, that is, deformationresistance is required for the base film of the membrane switch.Heretofore, a polyethylene terephthalate (may be abbreviated as "PET"hereinafter) film has been generally and frequently used as the basefilm of this membrane switch due to its deformation resistance, adhesionto electrodes, adhesion to printing and the like.

However, owing to the recent use of touch panels for car audio systemsand car air conditioning systems or remote control switches in a vehicledue to the popularization of car navigation systems, deformationresistance at high temperatures is required for the base film of themembrane switch.

When a PET film is used as the base film of the membrane switch asbefore, temperature inside a vehicle (about 80° C.) may exceed the glasstransition temperature of PET in summer and the membrane switch maymalfunction due to the great deformation of a PET film.

To prevent this, JP-B 4-75610 proposes that a polyethylene naphthalenedicarboxylate (may be abbreviated as "PEN" hereinafter) film which has ahigher glass transition temperature than a PET film be used as the basefilm in place of the PET film.

That is, JP-B 4-75610 discloses a membrane switch having contact pointson the opposed surfaces of two base films, at least one of which is abiaxially oriented polyethylene naphthalene dicarboxylate film having anF-5 value (5% elongation stress) of 11 kg/mm² or more, a density of1.375 g/cm³ or less and a thermal shrinkage factor of 1.0% or less whenheated at 120° C. for 30 minutes.

JP-B 6-4276 discloses a polyester film for a membrane switch which ismade from polyethylene naphthalate having a haze increase rate definedby the following expression of 20% or less when heated at 150° C. for 2hours:

    haze increase rate=(H.sub.2 -H.sub.1)/H.sub.1 ×100 (%)

wherein H₁ is a haze value before the heat treatment and H₂ is a hazevalue after the heat treatment, and thermal shrinkage factors in bothlongitudinal and transverse directions of 0.5% or less.

However, standards for deformation resistance have been reviewed due tothe influence of the recent enforcement of the PL Act and it has beenpointed out that even a PEN film is unsatisfactory in terms ofdeformation resistance. When a membrane switch is used as a pressuresensor, especially when it is embedded in a seat of an automobileequipped with an air bag as a pressure sensor of a weight detectionsystem for detecting pressure to alter the inflation speed of the airbag according to an adult or a child, a conventional PEN film has largeresidual deformation and cannot be used.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a biaxially orientedpolyester film for a membrane switch, which is made from a polyestercomprising ethylene-2,6-naphthalene dicarboxylate as a main recurringunit and which has excellent deformation resistance during its use athigh temperatures.

It is another object of the present invention to provide a biaxiallyoriented polyester film for a membrane switch, which is made from apolyester comprising ethylene-2,6-naphthalene dicarboxylate as a mainrecurring unit and which has excellent heat resistance, switchingperformance, dimensional stability, flexing resistance andprocessability.

It is still another object of the present invention to provide abiaxially oriented polyester film for use in a membrane switch used inan automobile, machine such as a heating vessel peripheral, or portableequipment where it may be exposed to high temperatures.

It is still another object of the present invention to provide a processfor producing the aforementioned biaxially oriented polyester film ofthe present invention.

It is still another object of the present invention to provide amembrane switch comprising the biaxially oriented polyester film of thepresent invention.

Other objects and advantages of the present invention will becomeapparent from the following description.

According to the present invention, firstly, the above objects andadvantages of the present invention are attained by a biaxially orientedpolyester film for a membrane switch,

(A) which is made from a polyester containing ethylene-2,6-naphthalenedicarboxylate recurring units in an amount of at least 80 mol %; and

(B) which has an endothermic peak with an endothermic energy of at least0.4 mJ/mg at a temperature of 110 to 160° C.

DETAILED DESCRIPTION OF THE INVENTION

The biaxially oriented polyester film for a membrane switch of thepresent invention comprises a polyester (A) which containsethylene-2,6-naphthalene dicarboxylate recurring units in an amount ofat least 80 mol %.

The polyester (A) comprehends an ethylene-2,6-naphthalene dicarboxylatehomopolymer and a copolymer containing ethylene-2,6-naphthalenedicarboxylate recurring units in an amount of at least 80 mol %.

Dicarboxylic acid components forming the copolymer other than2,6-naphthalenedicarboxylic acid are dicarboxylic acids such as oxalicacid, adipic acid, phthalic acid, sebacic acid, dodecanedicarboxylicacid, isophthalic acid, terephthalic acid, 1,4-cyclohexanedicarboxylicacid, 4,4'-diphenyldicarboxylic acid, phenylindanedicarboxylic acid,2,7-naphthalenedicarboxylic acid and diphenyl ether dicarboxylic acid.Diol components forming the copolymer other than ethylene glycol arediols such as propylene glycol, trimethylene glycol, tetramethyleneglycol, hexamethylene glycol, cyclohexanemethylene glycol, neopentylglycol, bisphenol sulfone adduct with ethylene oxide, bisphenol A adductwith ethylene oxide, diethylene glycol and polyethylene oxide glycol.

Oxycarboxylic acids such as p-oxybenzoic acid and p-oxyethoxybenzoicacid may also be contained as a copolymer component.

The copolymer may have terminal hydroxyl groups and/or carboxyl groupsthat have been partly or wholly terminated by a monofunctional compoundsuch as benzoic acid or methoxypolyalkylene glycol, or may be obtainedby copolymerizing a compound having at least three ester-formingfunctional groups such as glycerin or pentaerythritol in such a traceamount that a substantially linear polymer can be obtained.

The polyester (A) preferably contains ethylene-2,6-naphthalenedicarboxylate recurring units in an amount of at least 85 mol %, morepreferably at least 90 mol %.

The biaxially oriented polyester film of the present invention may be acomposition comprising the above polyester (A) and other organicpolymer.

Illustrative examples of the other organic polymer include knownpolyesters, polyamides, polyimides, polyether imides and polyalkylenes.Of these organic polymers, polyesters such as polyethyleneterephthalate, polyethylene isophthalate, polytrimethyleneterephthalate, polyethylene-4,4'-tetramethylenediphenyl dicarboxylate,polyethylene-2,7-naphthalene dicarboxylate,polytrimethylene-2,6-naphthalene dicarboxylate,polyneopentylene-2,6-naphthalene dicarboxylate andpoly(bis(4-ethyleneoxyphenyl)sulfone)-2,6-naphthalene dicarboxylate arepreferred from the viewpoint of compatibility with the polyester (A). Ofthese, polyethylene terephthalate, polyethylene isophthalate,polytrimethylene terephthalate, polytrimethylene-2,6-naphthalenedicarboxylate and poly(bis(4-ethyleneoxyphenyl)sulfone)-2,6-naphthalenedicarboxylate are particularly preferred.

As for the ratio of the polyester (A) to the other organic polymer, theother organic polymer is preferably contained in an amount of 20 partsor less by weight, more preferably 15 parts or less by weight based on100 parts by weight of the polyester (A).

In the present invention, the polyester (A) preferably has an intrinsicviscosity of 0.40 to 0.90 dl/g, more preferably 0.43 to 0.85 dl/g,particularly preferably 0.45 to 0.80 dl/g. When the intrinsic viscosityis lower than 0.40 dl/g, the obtained film becomes fragile, is readilyburred at cut ends, and may be cracked from part of a bur and brokenwhile carried from one step to another. On the other hand, when theintrinsic viscosity is higher than 0.90 dl/g, polymerization takes longwith a commonly used synthesis method, thereby reducing productivitydisadvantageously. Since dedicated equipment is required to carry out aspecial polymerization method (such as solid-phase polymerization),production cost rises, which is one of the reasons why the intrinsicviscosity of higher than 0.90 dl/g is undesirable.

The polyester (A) may contain such additives as a stabilizer, lubricant,ultraviolet absorber and flame retardant.

To provide slipperiness to the film, a small amount of inert fineparticles is preferably contained in the polyester (A). Illustrativeexamples of the inert fine particles include inorganic particles such asspherical silica, porous silica, calcium carbonate, alumina, titaniumdioxide, kaolin clay, barium sulfate and zeolite; and organic particlessuch as silicone resin particles and crosslinked polystyrene particles.Synthetic inorganic particles are more preferred than natural inorganicparticles because they are uniform in diameter, and inorganic particleshaving any crystal form, hardness, specific gravity and color may beused. One type or two or more different types of inert fine particlesselected from the above examples may be added to the film.

The average particle diameter of the above inert fine particles ispreferably in the range of 0.05 to 5.0 μm, more preferably 0.1 to 3.0μm. The content of the inert fine particles is preferably 0.001 to 1.0wt %, more preferably 0.03 to 0.5 wt %.

The time of adding the inert fine particles is not particularly limitedas long as it is before a film is formed from the polyester (A), forexample, in the stage of polymerization or at the time of filmformation.

The biaxially oriented polyester film of the present invention has anendothermic peak with an endothermic energy of at least 0.4 mJ/mg at atemperature of 110 to 160° C. The existence of this endothermic peak andthe quantity of the endothermic energy thereof are confirmed by adifferential scanning calorimeter (may be abbreviated as DSChereinafter). This endothermic peak is different from a peak indicatingthe heat of crystal fusion. Since the film is excellent in deformationresistance at high temperatures when both the temperature at which theendothermic peak appears and the quantity of the endothermic energy arewithin the above ranges, a membrane switch works properly during its useat high temperatures. The quantity of the endothermic energy ispreferably at least 0.5 mJ/mg, more preferably 0.5 to 4.0 mJ/mg. Theendothermic peak is preferably existent in the temperature range of 115to 155° C.

The biaxially oriented polyester film of the present inventionpreferably has a density of 1.345 to 1.370 g/cm³, more preferably 1.350to 1.365 g/cm³. When the density is smaller than 1.345 g/cm³, therecovery of the film from the deformation caused by alternatingdepressing and releasing a membrane switch formed of the film is smalland the membrane switch easily malfunctions disadvantageously. On theother hand, when the density is larger than 1.370 g/cm³, crystallinitybecomes too high with the result that the stiffness of the film is lost.Therefore, the film is not preferred as a base film for a membraneswitch.

The biaxially oriented polyester film of the present inventionpreferably has a deformation increase of 0.070% or less in a desireddirection, for example, longitudinal direction (MD) or transversedirection (TD) after placed under a load of 0.75 kg/mm² at 80° C. for 30hours. When the deformation increase is more than 0.070%, the plasticdeformation of the film becomes large at the time of depressing amembrane switch formed of the film at a high temperature, whereby theoperation accuracy of the membrane switch is liable to deteriorateduring its use at high temperatures. The deformation increase in boththe longitudinal direction (MD) and the transverse direction (TD) ismore preferably 0.065% or less, particularly preferably 0.060% or less.

The biaxially oriented polyester film of the present inventionpreferably has a residual deformation of 0.090% or less in a desireddirection, for example, longitudinal direction (MD) or transversedirection (TD) after it is placed under a load of 0.75 kg/mm² at 80° C.for 30 hours and after the load is removed. When the residualdeformation is larger than 0.090%, the deformation of the film remainseven after the load on a membrane switch formed of the film is removed,whereby the operation accuracy of the membrane switch is liable todeteriorate.

The residual deformation in both the longitudinal direction (MD) and thetransverse direction (TD) is more preferably 0.085% or less,particularly preferably 0.080% or less.

The biaxially oriented polyester film of the present inventionpreferably has a 1% elongation stress of 4.5 to 7.0 kg/mm² in a desireddirection, for example, longitudinal direction (MD) or transversedirection (TD).

When the 1% elongation stress is smaller than 4.5 kg/mm², non-uniformityin the thickness of the film tends to be large due to insufficient filmstiffness or insufficient stretching, thereby causing instableconditions for processing a membrane switch.

A film having a 1% elongation stress of more than 7.0 kg/mm² is brokeneasily and frequently during film formation, whereby film-formingproperties become unsatisfactory and productivity is apt to lower.

The 1% elongation stress in both the longitudinal direction (MD) and thetransverse direction (TD) is more preferably 4.7 to 6.8 (kg/mm²).

The biaxially oriented polyester film of the present inventionpreferably has Young's moduli of 400 to 700 kg/mm² in two directionsintersecting at a right angle, for example, longitudinal direction (MD)and transverse direction (D). When the Young's moduli are smaller than400 kg/mm², the stiffness of the film becomes insufficient.

When the Young's moduli are larger than 700 kg/mm², delamination islikely to occur at the time of cutting the film or chippings are liableto be produced in large quantity. Although the difference betweenYoung's moduli in both directions is not particularly limited, it ispreferably 150 kg/mm² or less.

The biaxially oriented polyester film of the present inventionpreferably has a refractive index in a thickness direction of 1.490 to1.530, more preferably 1.495 to 1.520. When the refractive index in thethickness direction is smaller than 1.490, burrs or cracks arefrequently formed by punching or cutting the film, whereby itsprocessability deteriorates disadvantageously. On the other hand, whenthe refractive index in the thickness direction is larger than 1.530,non-uniformity in the thickness of the film becomes large, wherebywrinkles (flutes) are readily formed on the surface of the filmdisadvantageously.

The biaxially oriented polyester film of the present inventionpreferably shows a moisture-vapor transmission of 0.03 to 0.35 g/m²·mm·24 hr, more preferably 0.05 to 0.30 g/m² ·mm·24 hr. When themoisture-vapor transmission is higher than 0.35 g/m² ·mm·24 hr, watervapor easily enters the switch. If water vapor enters the switch and theswitch is exposed to a high-temperature and high-humidity environmentand then to a low-temperature environment, dew may be condensed in theinside of the switch and the switch may malfunction disadvantageously.

The biaxially oriented polyester film of the present invention has athermal shrinkage factor of 0.20% or less, more preferably 0.15% orless, in a desired direction, for example, longitudinal direction ortransverse direction when heated at 150° C. for 30 minutes. When thethermal shrinkage factor is more than 0.20% after a 30-minute heattreatment at 150° C., the operation accuracy of the switch is liable todeteriorate due to a large dimensional change and the flatness of thefilm is also lowered disadvantageously.

The difference of thermal shrinkage factor between the two directionsintersecting at a right angle is preferably 0.10% or less to prevent areduction in flatness.

The biaxially oriented polyester film of the present inventionpreferably has a plane orientation coefficient of 0.242 to 0.270. Whenthe plane orientation coefficient is smaller than 0.242, non-uniformityin the thickness of the film becomes large, whereby wrinkles (flutes)are easily formed on the surface of the film. On the other hand, whenthe plane orientation coefficient is larger than 0.270, burrs or cracksare frequency formed by punching the film, thereby deterioratingprocessability.

The plane orientation coefficient is more preferably 0.245 to 0.265.

The biaxially oriented polyester film of the present inventionpreferably has a center line average surface roughness (Ra) of 5 to 200nm, more preferably 7 to 150 nm. When the surface roughness (Ra) issmaller than 5 nm, the slipperiness of the film degrades, wherebyblocking between films frequently occurs when the film is wound round aroll and scratches are frequently formed by a carrying roll during therunning of the film.

The biaxially oriented polyester film of the present inventionpreferably has a thickness of 12 to 250 μm. The thickness is morepreferably 25 to 250 μm, particularly preferably 50 to 200 μm, from theviewpoint of film strength and flexibility.

The biaxially oriented polyester film of the present invention has athickness non-uniformity of 10% or less. In other words, the thicknessnon-uniformity is 10% or less, that is, ±5% of the thickness of thefilm. When a film having a thickness non-uniformity of more than ±5% isused as a base material for a membrane switch, the intervals in betweenswitches do not become constant due to thickness non-uniformity even ifthe switches are designed and fabricated in the same way, whereby theoperation condition differs by each switch disadvantageously.Particularly when the film is used for a membrane switch for controllingthe inflation pressure of a car air bag, thickness non-uniformity is anextremely important factor because the malfunction of the switchdirectly affects people's lives.

The biaxially oriented polyester film of the present invention can beproduced by a process which comprises subjecting a biaxially orientedpolyester film produced from a polyester (A) containing at least 80 mol% of ethylene-2,6-naphthalene dicarboxylate recurring units inaccordance with a process known per se to the following steps in theorder named:

(1) the step of heating the film at a temperature of 150 to 180° C. for1 to 5 hours under no strain; and

(2) the step of heating the film in an unrolled or rolled state at atemperature of 80 to 122° C. for 5 to 200 hours.

The above process known per se includes, for example, one whichcomprises stretching an unstretched film of the polyester (A) to 2.0 to5.0 times, preferably to 2.2 to 4.3 times, in longitudinal andtransverse directions at a temperature of Tg to (Tg+60)° C. andheat-setting the biaxially oriented film at a temperature of (Tg+70) to(Tg+140)° C., preferably (Tg+80) to (Tg+137)° C., particularlypreferably (Tg+100) to (Tg+133)° C. for 1 to 100 sec. Stretching may becarried out with commonly used means using a roll or a stenter. The filmmay be stretched in longitudinal and transverse directionssimultaneously or sequentially.

A relaxation treatment may be further carried out. In this case, therelaxation treatment is preferably carried out after it is heat-set andbefore the film is wound round a roll. Relaxation treatment methodsinclude one which comprises cutting off both end portions of the film inthe heat-setting zone and making the take-up speed lower than thedelivery speed of the film at a temperature higher than Tg and lowerthan the melting temperature of the film, one which comprises heatingthe film with an IR heater between two carrying rolls having differentspeeds, one which comprises reducing the speed of a heating and carryingroll after the film is carried over the heating and carrying roll, onewhich comprises making the take-up speed lower than the delivery speedwhile the film is carried over a nozzle for blowing out hot air afterheat setting, one which comprises carrying the film over a heating andcarrying roll after the film is wound with a film forming machine andbefore a heat treatment at a temperature lower than Tg to reduce thespeed of the carrying roll, or one which comprises making the roll speedafter a heating zone lower than the roll speed before the heating zonewhile the film is carried through a heating oven or a heating zone withan IR heater. Any one of the above methods may be used. In any way, therelaxation treatment is carried out by making the take-up speed 0.1 to10% lower than the delivery speed.

The biaxially oriented polyester film produced by the above method knownper se is subjected to the above steps (1) and (2) in the order named.However, only the step (2) may be carried out when the thermal shrinkagefactor of the film which has been subjected to the above relaxationtreatment is 0.20% or less in both longitudinal and transversedirections after it is heated at a temperature of 150° C. for 30minutes.

In the above step (1), the film is heated under no strain at atemperature of 150 to 180° C. for 1 to 5 hours. After the heat treatmentof the step (1), the film is preferably cooled to a temperature of (Tgof film-40) to (Tg of film-5)° C. at a temperature reducing rate(gradual cooling) of 3 to 25° C./hr.

In the above step (2), the film which has been subjected to the step (1)or to the above relaxation treatment is further heated under no strainat a temperature of 80 to 122° C. for 5 to 200 hours.

This heat treatment is preferably carried out at a temperature of Tg(glass transition temperature of the polyester film) to (Tg-40)° C. Whenthe heat treatment temperature is lower than (Tg-40)° C., a very longheat treatment time is required to develop deformation resistance athigh temperatures, thereby reducing production efficiencydisadvantageously. On the other hand, when the heat treatmenttemperature is higher than Tg of the film, the film is not suitable foruse as a film for a membrane switch because it has low deformationresistance. The heat treatment time is preferably 5 to 200 hours, morepreferably 8 to 170 hours from the viewpoint of production efficiency.

The heat treatment of the step (2) may be carried out at any time in theproduction process of a membrane switch after the step (1). When thefilm is exposed to a temperature higher than the heat treatmenttemperature after the step (2), the effect of the heat treatment islargely lost. Therefore, the film is preferably handled at a temperaturelower than the heat treatment temperature after the step (2).

The step (2) may be carried out after the drying step after printing inthe production process of a membrane switch. When a heat treatment at atemperature higher than Tg of the film must be carried out in thisdrying step, the film is preferably gradually cooled from the heattreatment temperature of the drying step before the step (2). The heattreatment method of the step (1) is, for example, one which comprisesheating the film in an oven in an unrolled state, one which comprisesheating the film by carrying it over a heating roll or through a heatingzone with an IR heater at an extremely low speed, or the like. Themethod, however, is not limited to these.

The heat treatment method of the step (2) is, for example, one whichcomprises heating the film in an oven in a rolled or unrolled stateafter the film is formed, one which comprises heating the film in anoven in a rolled or unrolled state after electrodes or a spacer isplaced on the film, one which comprises heating the film in an ovenafter a switch is assembled, or the like. The method, however, is notlimited to these.

The biaxially oriented polyester film of the present invention can beused advantageously as a base film for a membrane switch.

The membrane switch produced using the biaxially oriented polyester filmof the present invention is embedded in a car seat, for example.Particularly in a car equipped with an air bag, the membrane switch isadvantageous when embedded in a seat to adjust the operation (especiallyinflation speed) of the air bag and more advantageous when used as amembrane switch having both a pressure sensor function and a startswitch function. That is, membrane switches, which are scattered under aseat or which fully cover the seated area of an ordinary adult, areturned on by switching to activate the inflation speed control system ofan air bag when a person is seated. The number of contacted switches orthe total contact area of switches is changed according to the weight ofa seated person, and each of the membrane switches has two functions,which are a start switch and a weight detection pressure sensor, in asystem which sets the inflation speed of an air bag by detecting thenumber of contacted switches or the contact area to estimate the weightof the seated person. Therefore, it is possible to adjust the inflationspeed of an air bag which works at the time of a car crash bydistinguishing between an adult and a child with the membrane switch ofthe present invention which has the function of a pressure sensor fordetecting the weight of a person.

EXAMPLES

The following examples are given to further illustrate the presentinvention.

(1) measurement of purity of ethylene-2,6-naphthalene dicarboxylate

After a film sample is dissolved in a measurement solvent (CDCl₃ : CF₃COOD=1:1), a ¹ H-NMR measurement is carried out and the purity ofethylene-2,6-naphthalene dicarboxylate is calculated from the integralratio of the obtained signals.

(2) glass transition temperature (Tg)

This is measured by the differential scanning calorimeter DSC220 ofSeiko Instruments Inc. under the following conditions.

temperature elevation rate: 20° C./min

amount of sample: 10 mg

measured in a nitrogen gas stream

The sample is heated and fused under the above conditions, quenched andmeasured again under the above conditions.

(3) endothermic peak temperature, quantity of endothermic energy

These are measured using the differential scanning calorimeter DSC220 ofSeiko Instruments Inc. under the following conditions.

temperature elevation rate: 20° C./min

amount of sample: 10 mg

measured in a nitrogen gas stream

The endothermic behavior of the film sample when it is heated and fusedunder the above conditions is analyzed with primary differentiation andsecondary differentiation to determine a peak temperature.

At this point, the quantity of endothermic energy is obtained from anarea on an endothermic side on the corresponding DSC chart. This area isan area on an endothermic side which is shifted toward the endothermicside from the base line by elevating temperature, shows an endothermicpeak by continuing the elevation of temperature, and then returns to thebase line. The area (A) is obtained by connecting the heat absorptionstart temperature position to the heat absorption end temperatureposition with a straight line. The area of In (indium) is measured withthe above amount of a sample under the same DSC measurement conditionsand the quantity of endothermic energy is obtained from the followingexpression based on the condition that this area (B) is 28.5 mJ/mg.

    quantity of endothermic energy=(A/B)×28.5 (mJ/mg)

(4) Young's moduli

The film is cut to a width of 10 mm and a length of 150 mm and thissample film is pulled by an Instron-type universal tensile tester at achuck interval of 100 mm, a pulling rate of 10 mm/min and a chart speedof 500 mm/min. Young's moduli are calculated from the tangent line of arising portion of the obtained load-elongation curve.

(5) refractive index in thickness direction

The refractive index in a film thickness direction is obtained using anNa-D ray and an Abbe refractometer (of Atago Co., Ltd.) at 25° C. Thefilm sample is measured for both front and rear sides, and the mean ofmeasurement values is taken as a refractive index in a thicknessdirection (nz).

(6) thermal shrinkage factor

The film is held in an oven maintained at 150° C. for 30 minutes underno strain, and the dimensional change of the film before and afterheating is calculated from the following expression as a thermalshrinkage factor.

    thermal shrinkage factor (%)=((L.sub.0 -L)/L.sub.0)×100

L₀ : distance between gauge marks before heating

L: distance between gauge marks after heating

(7) intrinsic viscosity

This is measured at 25° C. in o-chlorophenol as a solvent (unit: dl/g).

(8) folded-line delamination whitening ratio

The film is cut to a size of 80mm×80 mm, and the obtained film sample issoftly folded into two by hand, and pressed with a predeterminedpressure P1 (kg/cm²) by a press machine for 20 seconds. The pressed filmis then unfolded to its original shape by hand and pressed with apressure P2 (kg/cm²) for 20 seconds. Thereafter, the sample is taken outand the lengths (mm) of whitened portions appearing in the fold aremeasured and totaled.

New film samples are folded with P1=4.2, 6.3, 8.4 and 10.5 (kg/cm²),respectively, and unfolded with P2=1.26 (kg/cm²), and the abovemeasurement is made on each of the new film samples.

The proportion of the mean of the totaled lengths (mm) of whitenedportions at each pressure to the total length of the fold (80 mm) istaken as a folded-line delamination whitening ratio, and this value isused as an index indicating how easily the delamination of the filmoccurs.

    folded-line delamination whitening ratio (%)=(total of lengths of whitened portions (mm)/(80 mm×4))×100

(9) density

This is measured by a sink-float method at 25° C. in a density gradientobtained using an aqueous solution of calcium nitrate as a solvent.

(10) center line average surface roughness (Ra)

Both front and rear surfaces of the film are measured by a surfaceroughness meter (SURFCOM 111A of Tokyo Seimitsu Co., Ltd.) and the meanof measurement values is calculated and taken as a surface roughness.

(11) increase in deformation, amount of residual deformation

These are measured by the Thermo Mechanical Analyzer TMA/SS120C of SeikoInstruments Inc. under the following conditions.

temperature: 80° C.

load: 0.75 kg/mm²

load time: 30 hours

sample width: 4 mm

sample length (chuck interval): 15 mm

The above load is applied to the film sample at the above temperature,kept for a predetermined amount of time (30 hours) and then removed. Theamount of deformation (S₁) of the film when the load begins to beapplied, the amount of deformation (S₂) after the load is applied forthe predetermined amount of time, and the amount of deformation (S₃)immediately after the load is removed are read to obtain an increase indeformation after the load is applied for the predetermined amount oftime and the amount of residual deformation after the load is removed.The proportions of the obtained values to the length of the sample (S₀=chuck interval) before the application of the load are obtained fromthe following expressions and taken as an increase in deformation andthe amount of residual deformation.

    increase in deformation (%)=(S.sub.2 -S.sub.1)×100/S.sub.0

    amount of residual deformation (%)=S.sub.3 ×100/S.sub.0

(12) 1% elongation stress (F1 value)

The film is cut to a width of 10 mm and a length of 150 mm, and theobtained sample film is pulled by an Instron-type universal tensiletester at a chuck interval of 100 mm, a pulling rate of 10 mm/min and achart speed of 500 mm/min. The strength of the film at the time of 1%elongation is read from the load-elongation curve obtained by carryingout measurements in both MD and TD of the film and divided by theoriginal sectional area. The obtained value is taken as a 1% elongationstress (kg/mm²).

(13) plane orientation coefficient (ns)

The refractive indices in a longitudinal direction (MD) transversedirection (TD) and thickness direction (z) of the film are obtainedusing an Abbe refractometer (of Atago Co., Ltd.) and an Na-D ray at 25°C. The front and rear surfaces of the film sample are measured and themeans of measurement values are taken as refractive indices inlongitudinal, transverse and thickness directions. The birefringence andthe plane orientation coefficient (ns) are calculated from the followingtwo expressions.

    birefringence (An)=nMD-nTD

    plane orientation coefficient (ns)={(nMD+nTD)/2}-nz

(In these expressions, nMD is a refractive index in a longitudinaldirection, nTD is a refractive index in a transverse direction and nz isa refractive index in a thickness direction.)

(14) thickness and thickness non-uniformity

The thickness of the film is measured with the electronic micro-meter(K-312A) of Anritsu Co., Ltd. over a length of 2 m in both longitudinaland transverse directions at a needle pressure of 30 g and a runningspeed of 25 (mm/sec) to obtain a continuous thickness chart. The maximumthickness and the minimum thickness are read from this chart.

Further, the thickness of the same sample is calculated from its width(cm), length (cm), weight (g) and density (g/cm³) based on the followingexpression and taken as an average thickness.

The proportion of the difference between the maximum thickness and theminimum thickness to the average thickness is calculated from thefollowing two expressions and taken as thickness non-uniformity.

    average thickness (μm)=(weight/(width×length×density))×10,000

    thickness non-uniformity (%)=((maximum thickness-minimum thickness)/average thickness)×100

(15) evaluation on functionality of membrane switch

A predetermined pressure is applied to the membrane switch at 80° C. for96 hours. After the temperature is reduced to 25° C. with the abovepressure maintained, the pressure is removed. The measurement of eachsample is carried out at n=50. After the pressure is removed,probability (number of samples whose switch portions are separate fromeach other/50)×100%) is calculated from the number of samples whoseswitch portions are separate from each other with the switch portionsswitched off and evaluated based on the following three grades.

∘: good (probability of 95% or more) Δ: usable (probability of 80% more)

X: unusable (probability of less than 80%)

(16) film-forming properties

The film formation condition is observed to evaluate film-formingproperties based on the following criteria.

⊚: The film is not broken and very stable film formation is possible.

∘: The film is rarely broken and stable film formation is possible.

X: The film is often broken and film formation is instable.

(17) handling properties

The handling properties of the film are evaluated based on the followingcriteria.

∘: No blocking occurs and handling properties are satisfactory.

X: Blocking occurs and handling properties are unsatisfactory.

(18) processability

The processability of the film at the time of processing the film for amembrane switch, such as perforation, punchability and flatness isevaluated based on the following criteria.

⊚: Perforation and punchability are very good, the shape of the cut endof the film is good, and the flatness of the film after a conductivepaste or carbon paste is dried is high.

∘: The shape of the cut end of the film may be slightly bad, or theflatness of the film after a conductive paste or carbon paste is driedmay be low, but there is no practical problem.

X: Perforation and punchability are bad, the shape of the cut end isbad, or the flatness of the film after a conductive paste or carbonpaste is dried is low, so that a defective product is obtained.

Example 1

Polyethylene-2,6-naphthalene dicarboxylate, which contains 0.2 wt % ofsilica particles having an average particle diameter of 0.3 μm and whichhas an intrinsic viscosity of 0.60, was melt-extruded from a die slitand solidified by cooling on a casting drum to produce an unstretchedfilm.

This unstretched film was stretched to 3.6 times in a longitudinaldirection (mechanical axis direction) and to 3.8 times in a transversedirection (breadthwise direction) sequentially and heat set to produce a75-μm-thick biaxially oriented film,which was then wound round a roll.Thereafter, the rolled film was subjected to a relaxation treatment bymaking the roll speed after a heating zone 1% lower than the roll speedbefore the heating zone while carried through the heating zone with anIR heater.

After the obtained biaxially oriented film was left to stand at 150° C.for 30 minutes, it was found to have thermal shrinkage factors of 0.05%in a longitudinal direction and 0.02% in a transverse direction, anintrinsic viscosity of 0.55 dl/g, a density of 1.359 g/cm³, a refractiveindex in a thickness direction of 1.498 and Young's moduli inlongitudinal and transverse directions of 640 kg/mm².

A film sample of 1,000 mm in width and 2,000 m in length was cut out ofthe obtained biaxially oriented film and wound round a 165-mm-diameterroll to prepare a sample roll. The sample roll was subjected to a heattreatment by elevating the temperature to 115° C. over 24 hours,maintaining that temperature for 24 hours and reducing the temperatureto room temperature over 24 hours. It was observed by a DSC that theheat-treated film showed an endothermic peak as well as a peak showingheat of crystal fusion. The peak temperature was 135° C. and thequantity of endothermic energy was 1.0 mJ/mg.

A silver paste and a carbon paste were screen-printed on this PEN basefilm to form a conductor circuit and printed contact points(electrodes), respectively, and dried at 100° C. for 20 minutes toprepare a sheet for a switch. A film-shaped styrene-butadiene resin wasused as an adhesive to laminate two of the sheet together and as aspacer for a membrane switch.

Evaluation results on the physical properties of the biaxially orientedfilm and the functionality of the membrane switch are shown in Table 1.

Example 2

A film was formed in the same manner as in Example 1 to obtain amembrane switch except that a heat treatment was carried out at 115° C.for 8 hours after the formed film was wound round a roll. Evaluationresults are shown in Table 1.

Example 3

A film was formed in the same manner as in Example 1 to obtain amembrane switch except that a heat treatment was carried out at 95° C.for 48 hours after the formed film was wound round a roll. Evaluationresults are shown in Table 1.

Example 4

A membrane switch was obtained in the same manner as in Example 1 exceptthat the formed film was dried at 150° C. for 5 minutes with electrodesand a spacer placed on the film and, as a pre-heat treatment, graduallycooled from the drying temperature to 115° C. at a cooling rate of 15°C./hr and that a heat treatment was then carried out at 115° C. for 24hours. Evaluation results are shown in Table 1.

Example 5

A film was formed in the same manner as in Example 1 to obtain amembrane switch except that a copolyester having an intrinsic viscosityof 0.62 (a polyester comprising 90 mol % of ethylene-2,6-naphthalenedicarboxylate units and 10 mol % ofbis(4-(2-ethoxy)phenyl)sulfone-2,6-naphthalene dicarboxylate units(abbreviated as BPS-EO in the table) was used as a raw material.Evaluation results are shown in Table 1.

Example 6

A film was formed in the same manner as in Example 1 to obtain amembrane switch except that a copolyester having an intrinsic viscosityof 0.60 (a polyester comprising 95 mol % of ethylene-2,6-naphthalenedicarboxylate units and 5 mol % of ethylene-4,4'-diphenyl dicarboxylateunits (abbreviated as 4,4'-D in the table) was used as a raw material.Evaluation results are shown in Table 1.

Comparative Example 1

A film was formed in the same manner as in Example 1 to obtain amembrane switch except that a heat treatment was not carried out at atemperature lower than Tg of the film and higher than (Tg-40)° C. in anystage of the assembly of the membrane switch after the formed film waswound round a roll. Evaluation results are shown in Table 1. The filmwas poor in deformation resistance as a film for a membrane switch andthe functionality of the membrane switch was unsatisfactory.

Comparative Example 2

A film was formed in the same manner as in Example 1 to obtain amembrane switch except that a heat treatment was carried out at 70° C.for 100 hours after the formed film was wound round a roll. Evaluationresults are shown in Table 1. The film was unsatisfactory in terms ofdeformation resistance as a film for a membrane switch.

Comparative Example 3

A film was formed in the same manner as in Example 1 to obtain amembrane switch except that a heat treatment was carried out at 140° C.for 24 hours after the formed film was wound round a roll. Evaluationresults are shown in Table 1. The film was unsatisfactory in terms ofdeformation resistance as a film for a membrane switch.

                                      TABLE 1                                     __________________________________________________________________________                       Example.1                                                                          Example.2                                                                          Example.3                                                                          Example.4                                                                          Example.5                                                                          Example.6                         __________________________________________________________________________      main component polymer PEN PEN PEN PEN PEN PEN                                molar ratio of main component mol % 100 100 100 100 90 95                     copolymer component --  --  --  --  BPS-EO 4,4'-D                             molar ratio of copolymer component mol % -- -- -- -- 10 5                     type of lubricant spherical spherical spherical spherical spherical                                                     spherical                            silica silica silica silica silica silica                                    average particle diameter of lubricant pm 0.3 0.3 0.3 0.3 0.3 0.3                                                        amount of lubricant wt % 0.2                                                 0.2 0.2 0.2 0.2 0.2               stretch ratio                                                                       longitudinal direction (times)                                                             3.6  3.6  3.6  3.6  3.6  3.6                                  transverse direction (times) 3.8 3.8 3.8 3.8 3.8 3.8                       heat setting temperature ° C.                                                             Tg + 115                                                                           Tg + 115                                                                           Tg + 115                                                                           Tg + 115                                                                           Tg + 115                                                                           Tg + 115                            thickness μm 75 75 75 75 75 75                                             refractive index in thickness direction (nz) 1.498 1.498 1.498 1.498                                                    1.515 1.510                       young's                                                                             longitudinal direction kg/mm.sup.2                                                         640  640  640  640  450  490                                 modulus transverse direction kg/mm.sup.2 640 640 640 640 470 500            intrinsic viscosity dl/g                                                                         0.55 0.55 0.55 0.55 0.53 0.54                              __________________________________________________________________________       Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6                                        __________________________________________________________________________      density g/cm.sup.3 1.359 1.359 1.359 1.359 1.355 1.357                      thermal shrinkage factor                                                                 longitudinal                                                                          0.05 0.05 0.05 0.05 0.05 0.05                                (150° C. × 30 min) direction %                                    transverse 0.02 0.02 0.02 0.02 0.02 0.02                                      direction %                                                                folded-line delamination whitening ratio %                                                       50   50   50   50   0    0                                   glass transition temperature (Tg) ° C. 122 122 122 122 119 120                                                    endothermic peak temperature                                                 ° C. 135 125 120 145                                                   135 135                             quantity of endothermic energy mJ/mg 1.0 0.6 0.4 1.5 1.0 1.0                  heat treatment temperature ° C/hr 115 115 95 115 115 115                                                          heat treatment time hr 24 8                                                  48 24 24 24                         starting temperature for gradual cooling ° C. --  --  --  150                                                    --  --                              gradual cooling rate ° C./hr -- -- -- -15 -- --                      heat treatment timing                                                                            After the formed film was                                                                    After                                                                              After the formed                          wound up electrodes film was wound up                                          and a                                                                         spacer                                                                        are placed                                                                handling properties                                                                              ◯                                                                      ◯                                                                      ◯                                                                      ◯                                                                      ◯                                                                      ◯                       processability ⊚ ⊚ ⊚                                                       ⊚ .circleincirc                                                le. ⊚                evaluation on functionality of ◯ ◯ Δ                                                      ◯ ◯                                                   ◯                       membrane switch                                                               overall evaluation ⊚ ⊚ ◯                                                      ⊚ .circleincirc                                                le. ⊚                Ex.: Example                                                                __________________________________________________________________________                           Comparative                                                                             Comparative                                                                         Comparative                               Example.1 Example.2 Example.3                                              __________________________________________________________________________      main component polymer PEN PEN PEN                                            molar ratio of main component mol % 100 100 100                               copolymer component --  --  --                                                molar ratio of copolymer component mol % -- -- --                             type of lubricant spherical spherical spherical                                silica silica silica                                                         average particle diameter of lubricant μm 0.3 0.3 0.3                      amount of lubricant added wt % 0.2 0.2 0.2                                  stretch ratio                                                                           longitudinal direction (times)                                                             3.6       3.6   3.6                                       transverse direction (times) 3.8 3.8 3.8                                   heat setting temperature ° C.                                                                 Tg + 115  Tg+ 115                                                                             Tg + 115                                 thickness μm 75 75 75                                                      refractive index in thickness direction (nz) 1.498 1.498 1.498              young's   longitudinal direction kg/mm.sup.2                                                         640       640   640                                      modulus transverse direction kg/mm.sup.2 640 640 640                        intrinsic viscosity dl/g                                                                             0.55      0.55  0.55                                     density g/cm.sup.3 1.359 1.359 1.359                                        thermal shrinkage factor                                                                   longitudinal                                                                            0.05      0.05  0.05                                     (150° C. × 30 min) direction %                                    transverse direction % 0.02 0.02 0.02                                      folded-line delamination whitening ratio %                                                           50        50    50                                       glass transition temperature (Tg) ° C. 122 122 122                     endothermic peak temperature ° C. --  100 165                          quantity of endothermic energy mJ/mg -- 0.2 1.5                               heat treatment temperature ° C. -- 70 140                              heat treatment time hr -- 100 24                                              temperature for starting gradual cooling ° C. -- --  --                                                      gradual cooling rate °                                                C./hr -- -- --                         heat treatment timing  No treatment                                                                            After the formed film was                        wound up                                                                    handling properties ◯ ◯ ◯                 processability ⊚ ⊚ ⊚                                                           evaluation on functionalit                                                   y of membrane switch X X X       overall evaluation X X X                                                    __________________________________________________________________________

Example 7

After an ester interchange reaction was carried out between 100 parts ofdimethyl 2,6-naphthalene dicarboxylate and 60 parts of ethylene glycolin the presence of 0.03 part of manganese acetate tetrahydrate as anester interchange catalyst in accordance with a commonly used method byadding 0.2 wt % of spherical silica particles having an average particlediameter of 0.3 μm as a lubricant, 0.023 part of trimethyl phosphate wasadded to substantially terminate the ester interchange reaction.

Thereafter, 0.024 part of antimony trioxide was added and apolymerization reaction was carried out at a high temperature and a highdegree of vacuum in accordance with a commonly used method to producepolyethylene-2,6-naphthalene dicarboxylate (PEN Tg=122° C.) having anintrinsic viscosity of 0.62 dl/g. This silica particle-containing PENwas melt-extruded from a die slit and solidified by cooling on a castingdrum to produce an unstretched film.

This unstretched film was stretched to 3.5 times in a longitudinaldirection (mechanical axis direction) and to 3.7 times in a transversedirection (breadthwise direction) sequentially and heat set to produce a100-μm-thick biaxially oriented film, which was then wound round a roll.Thereafter, the rolled film was subjected to a relaxation treatment bymaking the roll speed after a heating zone 0.2% lower than the rollspeed before the heating zone while carried through the heating zonewith an IR heater.

The obtained biaxially oriented film was unrolled, sampled and subjectedto a heat treatment at 170° C. for 1 hour.

A silver paste and a carbon paste were screen-printed on this PEN basefilm to form a conductor circuit and printed contact points(electrodes), respectively, and dried at 100° C. for 20 minutes toproduce a sheet for a switch. A film-like styrene-butadiene resin wasused as an adhesive to laminate two of the sheet together and a spacerfor a membrane switch.

After the membrane switch is assembled, the switch was heated in an ovenmaintained at 115° C. for 24 hours.

Evaluation results on the physical properties of the biaxially orientedfilm and the functionality of the membrane switch are shown in Table 2.

Examples 8 and 9

Films were formed in the same manner as in Example 7 except that a heattreatment temperature or time after the assembly of a switch was changedas shown in Table 2. Results are shown in Table 2.

Example 10

A film was formed in the same manner as in Example 7 except that a heattreatment at a temperature lower than Tg was carried out before acircuit and electrodes were printed. Results are shown in Table 2.

Example 11

After a circuit and electrodes were printed and dried at 150° C. for 5minutes in Example 7, the temperature was gradually lowered from thedrying temperature to 115° C. at a cooling rate of 15° C./hr as apre-heat treatment and a heat treatment was further carried out at 115°C. for 24 hours. Results are shown in Table 2.

Examples 12 and 13

Films were formed in the same manner as in Example 7 except that filmforming conditions were changed as shown in Table 2. Results are shownin Table 2.

Example 14

A film was formed in the same manner as in Example 7 except that a 4%relaxation treatment was carried out only in a transverse direction at atemperature 15° C. lower than the heat setting temperature after theformed film was heat set. Film forming conditions and results are shownin Table 2.

Example 15

A film was formed in the same manner as in Example 7 except thatpolyethylene-2,6-naphthalene dicarboxylate (PEN Tg=120° C.), containingsilica particles and having an intrinsic viscosity of 0.54 dl/g, wasmelt-extruded from a die slit. Film forming conditions and results areshown in Table 2.

Example 16

A film was formed in the same manner as in Example 7 except that heatsetting temperature for forming the film was changed as shown in Table2. Film forming conditions and results are shown in Table 2.

Example 17

A film was formed in the same manner as in Example 7 except thatpolyethylene-2,6-naphthalene dicarboxylate (PEN Tg=122° C.), containing0. 3 wt % of spherical silica particles having an average particlediameter of 1.5 μm as a lubricant and having an intrinsic viscosity of0.62 dl/g, was used. Film forming conditions and results are shown inTable 2.

Example 18

After an ester interchange reaction was carried out among 89 parts ofdimethyl 2,6-naphthalene dicarboxylate, 11 parts of dimethylterephthalate (may be abbreviated as TA hereinafter) and 60 parts ofethylene glycol in the presence of 0.03 part of manganese acetatetetrahydrate as an ester interchange catalyst in accordance with acommonly used method by adding 0.2 wt % of spherical silica particleshaving an average particle diameter of 0.3 μm as a lubricant, 0.023 partof trimethyl phosphate was added to substantially terminate the esterinterchange reaction.

Thereafter, 0.024 part of antimony trioxide was added, and apolymerization reaction was carried out at a high temperature and a highdegree of vacuum in accordance with a commonly used method to producecopolyethylene-2,6-naphthalate (TA-copolymerized PEN Tg=113° C.) havingan intrinsic viscosity of 0.60 dl/g.

This copolyester was melt-extruded from a die slit as a raw material andsolidified by cooling on a casting drum to produce an unstretched film.This unstretched film was sequentially biaxially oriented in the samemanner as in Example 7 under the film forming conditions shown in Table2 to produce a biaxially oriented film. A membrane switch was fabricatedusing the obtained biaxially oriented film in the same manner as inExample 7 and heated at 105° C. for 24 hours. Results are shown in Table2.

                                      TABLE 2                                     __________________________________________________________________________                       Ex. 7 Ex. 8 Ex. 9 Ex. 10                                                                              Ex. 11                                                                              Ex. 12                       __________________________________________________________________________      main component polymer PEN PEN PEN PEN PEN PEN                                molar ratio of main component mol % 100 100 100 100 100 100                   copolymer component -- -- -- -- -- --                                         molar ratio of copolymer component mol % -- -- -- -- -- --                    type of lubricant spherical spherical spherical spherical spherical                                                          spherical                       silica silica silica silica silica silica                                    average particle diameter of lubricant μm 0.3 0.3 0.3 0.3 0.3 0.3                                                          amount of lubricant                                                          added wt % 0.2 0.2 0.2                                                        0.2 0.2 0.2                  stretch ratio                                                                       longitudinal direction (times)                                                             3.5   3.5   3.5   3.5   3.5   2.5                             transverse direction (times) 3.7 3.7 3.7 3.7 3.7 2.5                       heat setting temperature ° C.                                                             Tg + 120                                                                            Tg + 1.20                                                                           Tg + 120                                                                            Tg + 120                                                                            Tg + 120                                                                            Tg + 120                       thickness μm 100 100 100 100 100 100                                       thickness non-uniformity % 4 4 4 4 4 7                                      F1 value                                                                            longitudinal direction kg/mm.sup.2                                                         6.1   6.1   6.1   6.1   6.1   5.3                             transverse direction kg/mm.sup.2 6.2 6.2 6.2 6.2 6.2 5.3                   intrinsic viscosity dl/g                                                                         0.53  0.53  0.53  0.53  0.53  0.53                           density g/cm.sup.3 1.360 1.360 1.360 1.360 1.360 1.361                        surface roughness (Ra) nm 10 10 10 10 10 10                                   Ex.: Example                                                                __________________________________________________________________________       Example.7 Example.8 Example.9 Example.10 Example.11 Example.12             __________________________________________________________________________    increase in                                                                         longitudinal direction %                                                                   0.030 0.040 0.050 0.030 0.020 0.030                          deformation transverse direction % 0.030 0.040 0.050 0.030 0.020 0.030                                                        amount of longitudinal                                                       direction % 0.050 0.060                                                       0.070 0.050 0.040 0.050                                                        residual transverse                                                          direction %                    deformation  0.050 0.060 0.070 0.050 0.040 0.050                            glass transition temperature (Tg) ° C.                                                    1.22  122   122   122   122   122                            endothermic peak temperature ° C. 135 130 125 135 150 135                                                              quantity of endothermic                                                      energy mJ/mg 1.2 0.7 0.5                                                      1.2 2.5 1.2                    heat treatment temperature ° C. 115 115 100 115 115 115                heat treatment time hr 24 8 100 24 24 24                                      temperature for starting gradual cooling ° C. --  --  --  --                                                          150 --                         gradual cooling rate ° C./hr -- -- -- -- 15 --                         heat treatment timing after the after the after the before after after                                                       the                             assembly of assembly of assembly of printing of a printing and                                                              assembly of                     the switch the switch the switch circuit and drying of a the switch                                                             electrodes circuit                                                        and                                electrodes                                                                drying temperature after printing of a circuit 100 100 100 100 150 100                                                        and electrodes (°                                                      C.)                           film-forming property ⊚  ⊚  .circleincircl                                                     e.  ⊚                                                          ⊚                                                              ⊚                                                               processability .circlein                                                     circle. ⊚                                                      ⊚                                                              ⊚                                                              ⊚                                                              ⊚                                                               evaluation on functional                                                     ity of membrane .largecir                                                     cle. ◯                                                            ◯ .largecircl                                                     e. ◯                                                              ◯                  switch                                                                        overall evaluation ⊚ ⊚ ⊚                                                        ⊚                                                              ⊚                                                              ⊚                                                               Ex: Example                 __________________________________________________________________________       Ex. 13 Ex. 14 Ex. 15 Ex. 16 Ex. 17 Ex. 18                                  __________________________________________________________________________      main component polymer PEN PEN PEN PEN PEN PEN                                molar ratio of main component mol % 100 100 100 100 100 85                    copolymer component --  --  --  --  --  TA                                    molar ratio of copolymer component mol % -- -- -- -- -- 15                    type of lubricant spherical spherical spherical spherical spherical                                                          spherical                       silica silica silica silica silica silica                                    average particle diameter of lubricant μm 0.3 0.3 0.3 0.3 1.5 0.3                                                          amount of lubricant                                                          added wt % 0.2 0.2 0.2                                                        0.2 0.3 0.2                  stretch ratio                                                                       longitudinal direction (times)                                                             4.2   3.5   3.5   3.5   3.5   3.7                             transverse direction (times) 4.3 3.7 3.7 3.7 3.7 3.9                       heat setting temperature ° C.                                                             Tg + 120                                                                            Tg + 120                                                                            Tg + 120                                                                            Tg + 85                                                                             Tg + 120                                                                            Tg + 120                       thickness μm 100 100 100 100 100 100                                       thickness non-uniformity % 3 4 4 4 4 5                                      F1 value                                                                            longitudinal direction kg/mm.sup.2                                                         6.5   6.1   6.2   6.3   6.1   5.5                             transverse direction kg/mm.sup.2 6.5 6.1 6.2 6.3 6.2 5.6                   intrinsic viscosity dl/g                                                                         0.53  0.53  0.46  0.53  0.53  0.51                           density g/cm.sup.3 1.359 1.360 1.361 1.352 1.361 1.355                        surface roughness (Ra) nm 10 10 10 10 80 10                                   Ex.: Example                                                                __________________________________________________________________________       Example.13 Example.14 Example.15 Example.16 Example.17 Example.18          __________________________________________________________________________    increase in                                                                         longitudinal direction %                                                                   0.030 0.035 0.030 0.060 0.030 0.067                          deformation transverse direction % 0.030 0.030 0.030 0.060 0.036 0.067                                                        amount of longitudinal                                                       direction % 0.050 0.055                                                       0.048 0.080 0.050 0.085                                                        residual transverse                                                          direction % 0.050 0.050                                                       0.048 0.080 0.050 0.085                                                        deformation                 glass transition temperature (Tg) ° C.                                                    122   122   120   122   122   113                            endothermic peak temperature ° C. 135 135 135 133 135 125                                                              quantity of endothermic                                                      energy mJ/mg 1.0 1.1 1.3                                                      0.7 1.2 1.4                    heat treatment temperature ° C. 115 115 115 115 115 105                heat treatment time hr 24 24 24 24 24 24                                      temperature for starting gradual cooling ° C. --  --  --  --                                                          --  --                         gradual cooling rate ° C./hr -- -- -- -- -- --                         heat treatment timing after the after the after the after the after the                                                      after the                       assembly of assembly of assembly of assembly of assembly of assembly                                                        of                              the switch the switch the switch the switch the switch the switch                                                            drying temperature                                                           after printing of a                                                           circuit 100 100 100 100                                                       100 100                        and electrodes (° C.)                                                  film-forming property ⊚  ⊚  .circleincircl                                                     e.  ⊚                                                          ⊚                                                              ◯                  processability ⊚ ◯ ⊚ .largecir                                                     cle. ⊚                                                         ⊚                                                               evaluation on functional                                                     ity of membrane .largecir                                                     cle. ◯                                                            ◯ .largecircl                                                     e. ◯ Δ       switch                                                                        overall evaluation ⊚ ◯ ⊚                                                           ◯ .circleinci                                                     rcle. ◯          __________________________________________________________________________

Example 19

After an ester interchange reaction was carried out between 100 parts ofdimethyl 2,6-naphthalene dicarboxylate and 60 parts of ethylene glycolin the presence of 0.03 part of manganese acetate tetrahydrate as anester interchange catalyst in accordance with a commonly used method byadding 0.25 wt % of silica particles having an average particle diameterof 0.3 μm as a lubricant, 0.023 part of trimethyl phosphate was added tosubstantially terminate the ester interchange reaction.

Thereafter, 0.024 part of antimony trioxide was added, and apolymerization reaction was carried out at a high temperature and a highdegree of vacuum in accordance with a commonly used method to producepolyethylene-2,6-naphthalene dicarboxylate (PEN Tg=121° C.) having anintrinsic viscosity of 0.62 dl/g. This silica particle-containing PENwas melt-extruded from a die slit and solidified by cooling on a castingdrum to produce an unstretched film.

This unstretched film was stretched to 3.5 times in a longitudinaldirection (mechanical axis direction) and to 3.6 times in a transversedirection (breadthwise direction) sequentially and heat set to produce a75-μm-thick biaxially oriented film, which was then wound round a roll.Thereafter, the rolled film was subjected to a relaxation treatment bymaking the roll speed after a heating zone 0.3% lower than the rollspeed before the heating zone while carried through the heating zone(185° C.) with an IR heater for the purpose of increasing thedimensional stability of the film and facilitating the exhibition of aneffect by heat treatment at a temperature lower than Tg. A film sampleof 1,000 mm in width and 2,000 m in length was cut out of the obtainedbiaxially oriented film and wound round a 165-mm-diameter roll toprepare a sample roll. This sample roll was heated in an oven maintainedat 115° C. for 24 hours to produce a film for a membrane switch.

A silver paste and a carbon paste were screen-printed on this PEN basefilm to form a conductor circuit and printed contact points(electrodes), respectively, and dried at 110° C. for 20 minutes toproduce a sheet for a switch. A film-like styrene-butadiene resin wasused as an adhesive to laminate two of the sheet together and as aspacer for a membrane switch.

Evaluation results on the physical properties of the biaxially orientedfilm and the functionality of the membrane switch are shown in Table 3.

Examples 20 to 23

Films were formed and a heat treatment was carried out in the samemanner as in Example 19 except that film forming conditions were changedas shown in Table 3. Results are shown in Table 3.

Examples 24 and 25

Films were formed and a heat treatment was carried out in the samemanner as in Example 19 except that the types and amounts of lubricantsadded were changed. Results are shown in Table 3.

Example 26

A film was formed and a heat treatment was carried out in the samemanner as in Example 19 except that film forming conditions were changedas shown in Table 3 and that a 4% relaxation treatment was carried outonly in a transverse direction at a temperature 10° C. lower than theheat setting temperature after heat setting. Film forming conditions andresults are shown in Table 3.

Example 27

A film was formed in the same manner as in Example 19 except that theheat treatment time at 115° C. was changed to 7 hours. Results are shownin Table 3.

Example 28

After an ester interchange reaction was carried out among 89 parts ofdimethyl 2,6-naphthalene dicarboxylate, 11 parts of dimethylisophthalate (may be abbreviated as IA hereinafter) and 60 parts ofethylene glycol in the presence of 0.03 part of manganese acetatetetrahydrate as an ester interchange catalyst in accordance with acommonly used method by adding 0.25 wt % of silica particles having anaverage particle diameter of 0.3 μm as a lubricant, 0.023 part oftrimethyl phosphate was added to substantially terminate the esterinterchange reaction.

Thereafter, 0.024 part of antimony trioxide was added, and apolymerization reaction was carried out at a high temperature and a highdegree of vacuum in accordance with a commonly used method to producecopolyethylene-2,6-naphthalate (IA-copolymerized PEN Tg=115° C.) havingan intrinsic viscosity of 0.61 dl/g.

This co-PET was melt-extruded from a die slit and solidified by coolingon a casting drum to produce an unstretched film. The unstretched filmwas sequentially biaxially oriented in the same manner as in Example 19under the film forming conditions shown in Table 3 to produce abiaxially oriented film. The biaxially oriented film was then subjectedto a relaxation treatment and a heat treatment at 115° C. in the samemanner as in Example 19. Results are shown in Table 3.

                                      TABLE 3                                     __________________________________________________________________________                       Ex. 19                                                                              Ex. 20                                                                              Ex. 21                                                                              Ex. 22                                                                              Ex. 23                             __________________________________________________________________________      main component polymer PEN PEN PEN PEN PEN                                    molar ratio of main component mol % 100 100 100 100 100                       copolymer component --  --  --  --  --                                        molar ratio of copolymer component mol % -- -- -- -- --                       glass transition temperature of polymer 121 121 121 121 121                   (° C.)                                                                 type of lubricant spherical spherical spherical spherical spherical                                                      silica silica silica silica                                                 silica                               average particle diameter of lubricant μm 0.3 0.3 0.3 0.3 0.3                                                        amount of lubricant added wt                                                 % 0.25 0.25 0.25 0.25 0.25         stretch ratio                                                                       longitudinal direction (times)                                                             3.5   2.8   4.4   3.5   3.3                                   transverse direction (times) 3.6 3.0 4.5 3.6 3.4                           heat setting temperature (° C.)                                                           Tg + 118                                                                            Tg + 118                                                                            Tg + 118                                                                            Tg + 135                                                                            Tg + 90                              thickness μm 75 75 75 75 75                                                thickness non-uniformity % 4 6 3 6 3                                          refractive index in thickness direction (nz) 1.498 1.510 1.495 1.506                                                   1.495                                Ex.: Example                                                                __________________________________________________________________________       Example.19 Example.20 Example.21 Example.22 Example.23                     __________________________________________________________________________    F1 value                                                                            longitudinal direction kg/mm.sup.2                                                         3.3   2.9   3.5   3.2   3.5                                  at 80° C. transverse direction kg/mm.sup.2 3.4 3.0 3.6 3.2 3.5                                                   breaking longitudinal                                                        direction % 80 110 65 110 100                                                  extension transverse direction                                                % 90 130 65 110 120               intrinsic viscosity dl/g                                                                         0.54  0.54  0.54  0.54  0.54                                 density g/cm.sup.3 1.360 1.361 1.359 1.366 1.354                            thermal shrinkage factor                                                                  longitudinal                                                                         0.15  0.10  0.25  0.02  0.20                                 (150° C. × 30 min) direction %                                    transverse 0.05 0.05 0.10 0.01 0.10                                           direction %                                                                surface roughness (Ra) nm                                                                        15    15    15    15    15                                   moisture-vapor transmission g/m.sup.2 mm 24 hr 0.17 0.17 0.17 0.13 0.21       endothermic peak temperature ° C. 130 133 128 135 129                  quantity of endothermic energy mJ/mg 1.1 1.2 0.9 1.2 1.0                      film-forming properties ⊚  ⊚  .largecircle                                               .  ⊚  .circleinc                                               ircle.                               handling properties ◯ ◯ ◯ .largecirc                                               le. ◯                    processability ⊚ ⊚ ◯ .largecir                                               cle. ◯                   evaluation on functionality of membrane ◯ ◯                                                    ◯ ◯                                                   ◯                        switch                                                                        overall evaluation ⊚ ⊚ ◯                                                     ◯ ◯        __________________________________________________________________________       Ex. 24 Ex. 25 Ex. 26 Ex. 27 Ex. 28                                         __________________________________________________________________________      main component polymer PEN PEN PEN PEN PEN                                    molar ratio of main component mol % 100 100 100 100 85                        copolymer component --  --  --  --  IA                                        molar ratio of copolymer component mol % -- -- -- -- 15                       glass transition temperature of polymer 121 121 121 121 115                   (° C.)                                                                 type of lubricant spherical spherical spherical spherical spherical                                                      silica silica silica silica                                                 silica                               average particle diameter of lubricant μm 1.5 2.5 0.3 0.3 0.3                                                        amount of lubricant added wt                                                 % 0.4 0.3 0.25 0.25 0.25           stretch ratio                                                                       longitudinal direction (times)                                                             3.5   3.5   3.4   3.5   3.5                                   transverse direction (times) 3.6 3.6 3.5 3.6 3.6                           heat setting temperature (° C.)                                                           Tg + 118                                                                            Tg + 118                                                                            Tg + 125                                                                            Tg + 118                                                                            Tg + 118                             thickness μm 75 75 75 75 75                                                thickness non-uniformity % 4 4 4 4 6                                          refractive index in thickness direction (nz) 1.498 1.498 1.501 1.498                                                   1.513                                Ex.: Example                                                                __________________________________________________________________________       Example.24 Example.25 Example.26 Example.27 Example.28                     __________________________________________________________________________    F1 value                                                                            longitudinal direction kg/mm.sup.2                                                         3.3   3.3   3.2   3.3   2.9                                  at 80° C. transverse direction kg/mm.sup.2 3.4 3.4 3.3 3.4 2.9                                                   breaking longitudinal                                                        direction % 80 80 90 80 130                                                    extension transverse direction                                                % 90 90 100 90 140                intrinsic viscosity dl/g                                                                         0.54  0.54  0.54  0.54  0.51                                 density g/cm.sup.3 1.360 1.360 1.361 1.360 1.356                            thermal shrinkage factor                                                                  longitudinal                                                                         0.15  0.15  0.20  0.15  0.15                                 (150° C. × 30 min) direction %                                    transverse 0.05 0.05 0.05 0.05 0.10                                           direction %                                                                surface roughness (Ra) nm                                                                        90    150   15    15    15                                   moisture-vapor transmission g/m.sup.2 mm 24 hr 0.17 0.17 0.17 0.17 0.19       endothermic peak temperature ° C. 130 130 131 124 130                  quantity of endothermic energy mJ/mg 1.1 1.1 0.8 0.4 1.4                      film-forming properties ⊚  ⊚  .circleincir                                               cle.  ⊚                                                        ◯                        handling properties ◯ ◯ ◯ .largecirc                                               le. ◯                    processability ⊚ ⊚ ◯ .circlein                                               circle. ⊚                                                       evaluation on functionality                                                  of membrane ◯                                                     ◯ ◯                                                   Δ Δ                      switch                                                                        overall evaluation ⊚ ⊚ ◯                                                     ◯ ◯        __________________________________________________________________________

Example 29

After an ester interchange reaction was carried out between 100 parts ofdimethyl 2,6-naphthalene dicarboxylate and 60 parts of ethylene glycolin the presence of 0.03 part of manganese acetate tetrahydrate as anester interchange catalyst in accordance with a commonly used method byadding 0.25 wt % of silica particles having an average particle diameterof 0.3 μm as a lubricant, 0.023 part of trimethyl phosphate was added tosubstantially terminate the ester interchange reaction.

Thereafter, 0.024 part of antimony trioxide was added, and apolymerization reaction was carried out at a high temperature and a highdegree of vacuum in accordance with a commonly used method to producepolyethylene-2,6-naphthalene dicarboxylate (PEN Tg=121° C.) having anintrinsic viscosity of 0.62 dl/g. This silica particle-containing PENwas melt-extruded from a die slit and solidified by cooling on a castingdrum to produce an unstretched film.

This unstretched film was stretched to 3.4 times in a longitudinaldirection (mechanical axis direction) and to 3.5 times in a transversedirection (breadthwise direction) sequentially and heat set to produce a75-μm-thick biaxially oriented film, which was then wound round a roll.Thereafter, the rolled film was subjected to a relaxation treatment bymaking the roll speed after a heating zone 0.2% lower than the rollspeed before the heating zone while carried through the heating zone(185° C.) with an IR heater for the purpose of increasing thedimensional stability of the film and facilitating the exhibition of aneffect by heat treatment at a temperature lower than Tg. A film sampleof 1,000 mm in width and 2,000 m in length was cut out of the obtainedbiaxially oriented film and wound round a 165-mm-diameter roll toprepare a sample roll. The sample roll was subjected to a heat treatmentby elevating the temperature to 110° C. over 24 hours, maintaining thattemperature for 24 hours and reducing the temperature to roomtemperature over 24 hours.

A silver paste and a carbon paste were screen-printed on this PEN basefilm to form a conductor circuit and printed contact points(electrodes), respectively, and dried at 110° C. for 20 minutes toproduce a sheet for a switch. A film-like styrene-butadiene resin wasused as an adhesive to laminate two of the sheet together and as aspacer for a membrane switch.

Evaluation results on the physical properties of the biaxially orientedfilm and the functionality of the membrane switch are shown in Table 4.

Examples 30 to 33

Films were formed and a heat treatment was carried out in the samemanner as in Example 29 except that film forming conditions were changedas shown in Table 4. Results are shown in Table 4.

Example 34

After an ester interchange reaction was carried out among 89 parts ofdimethyl 2,6-naphthalene dicarboxylate, 11 parts of dimethylterephthalate (may be abbreviated as TA hereinafter) and 60 parts ofethylene glycol in the presence of 0.03 part of manganese acetatetetrahydrate as an ester interchange catalyst in accordance with acommonly used method by adding 0.25 wt % of silica particles having anaverage particle diameter of 0.3 μm as a lubricant, 0.023 part oftrimethyl phosphate was added to substantially terminate the esterinterchange reaction.

Thereafter, 0.024 part of antimony trioxide was added, and apolymerization reaction was carried out at a high temperature and a highdegree of vacuum in accordance with a commonly used method to producecopolyethylene-2,6-naphthalate (TA-copolymerized PEN Tg=113° C.) havingan intrinsic viscosity of 0.61 dl/g.

This copolyester was melt-extruded from a die slit and solidified bycooling on a casting drum to produce an unstretched film. Theunstretched film was sequentially biaxially oriented in the same manneras in Example 29 under the film forming conditions shown in Table 4 toproduce a biaxially oriented film. Thereafter, the biaxially orientedfilm was subjected to a relaxation treatment while carried through aheating zone with an IR heater and then to a heat treatment at110° C. inthe same manner as in Example 29. Results are shown in Table 4.

                                      TABLE 4                                     __________________________________________________________________________                          Ex. 29                                                                              Ex. 30                                                                             Ex. 31                                                                             Ex. 32                                                                             Ex. 33                                                                             Ex. 34                        __________________________________________________________________________    main component polymer                                                                              PEN   PEN  PEN  PEN  PEN  PEN                             molar ratio of main component mol % 100 100 100 100 100 85                    copolymer component --  --  --  --  --  TA                                    molar ratio of copolymer component mol % -- -- -- -- -- 15                    glass transition temperature of polymer (° C.) 121 121 121 121                                                       121 113                         type of lubricant spherical spherical spherical spherical spherical                                                         spherical                        silica silica silica silica silica silica                                    average particle diameter of lubricant μm 0.3 0.3 0.3 0.3 0.3 0.3                                                         amount of lubricant                                                          added wt % 0.25 0.25 0.25                                                     0.25 0.25 0.25                stretch ratio                                                                            longitudinal direction                                                                   3.4   3.3  3.5  3.4  3.4  3.5                              (times)                                                                       transverse direction 3.5 4.1 3.5 3.5. 3.5 3.6                                 (times)                                                                    heat setting temperature (° C.)                                                              Tg + 115                                                                            Tg + 115                                                                           Tg + 115                                                                           Tg + 105                                                                           Tg + 130                                                                           Tg + 115                        thickness μm 75 75 75 75 75 75                                             thickness non-uniformity % 4 4 4 3 4 6                                        birefringence (Δn) -0.020 -0.065 0.015 -0.025 -0.008 -0.012                                                            plane orientation                                                            coefficient (ns) 0.255                                                        0.261 0.256 0.252 0.249                                                       0.248                         young's    longitudinal direction                                                                   585   570  600  600  560  520                             modulus kg/mm.sup.2                                                            transverse direction 590 640 590 615 575 540                                  kg/mm.sup.2                                                                intrinsic viscosity dl/g                                                                            0.54  0.54 0.54 0.54 0.54 0.51                            density g/m.sup.3 1.358 1.359 1.358 1.356 1.362 1.354                       thermal shrinkage factor                                                                 longitudinal direction %                                                                 0.05  0.05 0.05 0.13 0.03 0.15                            (150° C. × 30 min) transverse direction % 0.01 0.01 0.01                                                       0.05 0.01 0.19                surface roughness (Ra) nm                                                                           15    15   15   15   15   15                              moisture-vapor transmission g/m.sup.2 mm 24 hr 0.18 0.18 0.18 0.20 0.14                                                     0.22                            endothermic peak temperature ° C. 1.30 130 130 130 130 130                                                            quantity of endothermic                                                      energy mJ/mg 1.0 0.8 1.0                                                      1.0 1.2 1.5                     handling properties ◯ ◯ ◯ .largecirc                                                    le. ◯                                                             ◯                   processability ⊚ ⊚ ⊚                                                           ◯ .circleincir                                                    cle. ⊚                                                          evaluation on functionali                                                    ty of membrane switch                                                         ◯ .largecircle                                                    . ◯ .largecirc                                                    le. ◯ Δ       overall evaluation ⊚ ⊚ ⊚                                                       ⊚ .circlein                                                    circle. ◯         __________________________________________________________________________     Ex.: Example                                                             

Example 35

After an ester interchange reaction was carried out between 100 parts ofdimethyl 2,6-naphthalene dicarboxylate and 60 parts of ethylene glycolin the presence of 0.03 part of manganese acetate tetrahydrate as anester interchange catalyst in accordance with a commonly used method byadding 0.2 wt % of silica particles having an average particle diameterof 0.3 μm as a lubricant, 0.023 part of trimethyl phosphate was added tosubstantially terminate the ester interchange reaction.

Thereafter, 0.024 part of antimony trioxide was added, and apolymerization reaction was carried out at a high temperature and a highdegree of vacuum in accordance with a commonly used method to producepolyethylene-2,6-naphthalene dicarboxylate (PEN Tg=121° C.) having anintrinsic viscosity of 0.62 dl/g. This silica particle-containing PENwas melt-extruded from a die slit and solidified by cooling on a castingdrum to produce an unstretched film.

After the unstretched film was stretched to 3.3 times in a longitudinaldirection (mechanical axis direction) and to 3.4 times in a transversedirection (breadthwise direction) sequentially while the same quantityof heat was applied to the front and rear sides of the film, thestretched film was heat set to produce a 75-μm-thick biaxially orientedfilm, which was then wound round a roll. Thereafter, the rolled film wassubjected to a relaxation treatment by making the roll speed after aheating zone 0.2% lower than the roll speed before the heating zonewhile carried through the heating zone with an IR heater. The obtainedfilm was heated in an unrolled state at 170° C. for 2 hours and then at105° C. for 16 hours.

A silver paste and a carbon paste were screen-printed on this PEN basefilm to form a conductor circuit and printed contact points(electrodes), respectively, and dried at 100° C. for 20 minutes toproduce a sheet for a switch. A film-like styrene-butadiene resin wasused as an adhesive to laminate two of the sheet together and as aspacer for a membrane switch.

Evaluation results on the physical properties of the biaxially orientedfilm and the functionality of the membrane switch are shown in Table 5.

Example 36

A film was formed in the same manner as in Example 35 except that theheat treatment time at 105° C. was changed to 8 hours, and the formedfilm was subjected to a relaxation treatment and a heat treatment at170° C. Results are shown in Table 5.

Example 37

A film was formed in the same manner as in Example 35 except that filmforming conditions were changed as shown in Table 5, and the formed filmwas subjected to a relaxation treatment, a heat treatment at 170° C. anda heat treatment at 105° C. Results are shown in Table 5.

Example 38

A film was formed in the same manner as in Example 35 except that thefilm was stretched in a longitudinal direction with the quantity of heat1.6 times as large as that applied to its rear side applied to its frontside and that other film forming conditions were changed as shown inTable 5, and the formed film was subjected to a relaxation treatment, aheat treatment at 170° C. and a heat treatment at 105° C. Results areshown in Table 5.

Example 39

After an ester interchange reaction was carried out among 89 parts ofdimethyl 2,6-naphthalene dicarboxylate, 11 parts of dimethylterephthalate (may be abbreviated as TA hereinafter) and 60 parts ofethylene glycol in the presence of 0.03 part of manganese acetatetetrahydrate as an ester interchange catalyst in accordance with acommonly used method by adding 0.2wt % of silica particles having anaverage particle diameter of 0.3 μm as a lubricant, 0.023 part oftrimethyl phosphate was added to substantially terminate the esterinterchange reaction.

Thereafter, 0.024 part of antimony trioxide was added, and apolymerization reaction was carried out at a high temperature and a highdegree of vacuum in accordance with a commonly used method to producecopolyethylene-2,6-naphthalate (TA-copolymerized PEN Tg=113° C.) havingan intrinsic viscosity of 0.61 dl/g.

This copolyester was melt-extruded from a die slit as a raw material andsolidified by cooling on a casting drum to produce an unstretched film.The unstretched film was sequentially biaxially oriented in the samemanner as in Example 37 under the film forming conditions shown in Table5 to produce a biaxially oriented film. Thereafter, the film wassubjected to a relaxation treatment while carried through a heating zonewith an IR heater in the same manner as in Example37. The obtained filmwas heated in an unrolled state at 170° C. for 2 hours and then at 105°C. for 16 hours. Results are shown in Table 5.

                                      TABLE 5                                     __________________________________________________________________________                          Ex. 35                                                                             Ex. 36                                                                             Ex. 37                                                                             Ex. 38                                                                             Ex. 39                              __________________________________________________________________________    main component polymer                                                                              PEN  PEN  PEN  PEN  PEN                                   molar ratio of main component mol % 100 100 100 100 85                        copolymer component --  --  --  --  TA                                        molar ratio of copolymer component mol % -- -- -- -- 15                       glass transition temperature of polymer (° C.) 121 121 121 121                                                 113                                   type of lubricant spherical spherical spherical spherical spherical                                                     silica silica silica silica                                                 silica                                average particle diameter of lubricant μm 0.3 0.3 0.3 0.3 0.3                                                       amount of lubricant wt % 0.2                                                 0.2 0.2 0.2 0.2                     stretch ratio                                                                            longitudinal direction                                                                   3.3  3.3  2.8  3.5  3.6                                    (times)                                                                       transverse direction 3.4 3.4 3.0 3.6 3.8                                      (times)                                                                    heat setting temperature (° C.)                                                              Tg + 105                                                                           Tg + 105                                                                           Tg + 125                                                                           Tg + 105                                                                           Tg + 105                              thickness μm 75 75 75 75 75                                                thickness non-uniformity % 4 4 6 4 5                                          difference in absolute value between plane 0.002 0.002 0.001 0.005                                                    0.001                                 orientation coefficient of front side & that of                               rear side                                                                     |ns on front side - ns on rear side|                      increase in deformation                                                                  longitudinal direction %                                                                 0.035                                                                              0.045                                                                              0.035                                                                              0.030                                                                              0.070                                  transverse 0.035 0.045 0.035 0.030 0.070                                      direction %                                                                  young's longitudinal direction 600 600 550 630 540                            modulus kg/mm.sup.2                                                            transverse direction 620 620 570 640 550                                      kg/mm.sup.2                                                                intrinsic viscosity dl/g                                                                            0.54 0.54 0.54 0.54 0.51                                  density g/cm.sup.3 1.358 1.358 1.361 1.358 1.352                            thermal shrinkage factor                                                                 longitudinal direction %                                                                 0.05 0.05 0.03 0.15 0.15                                  (150° C. × 30 min) transverse direction % 0.01 0.01 0.01                                                 0.03 0.03                           surface roughness (Ra) nm                                                                           10   10   10   10   10                                    moisture-vapor transmission g/m.sup.2 mm 24 hr 0.16 0.16 0.14 0.16 0.20       endothermic peak temperature ° C. 125 120 125 125 125                  quantity of endothermic energy mJ/mg 0.7 0.5 0.8 0.6 0.8                      handling properties ◯  ◯  ◯                                                       ◯  ◯                                                   processability ⊚                                               ⊚ .circleincircl                                              e. ◯ .circleincircle                                              .                                     evaluation on functionality of membrane switch ◯ .largecircl                                              e. ◯ ◯                                                Δ                               overall evaluation ⊚ ⊚ ⊚                                                 ◯ ◯         __________________________________________________________________________     Ex.: Example                                                             

What is claimed is:
 1. A biaxially oriented polyester film for amembrane switch,(A) which comprises a polyester containing at least 80mol % of ethylene-2,6-naphthalene dicarboxylate recurring units, and (B)which has an endothermic peak with an endothermic energy of at least 0.4mJ/mg at a temperature of 110 to 160° C.
 2. The biaxially orientedpolyester film of claim 1, wherein the polyester (A) contains at least85 mol % of ethylene-2,6-naphthalene dicarboxylate recurring units. 3.The biaxially oriented polyester film of claim 1, wherein the polyester(A) has an intrinsic viscosity of 0.40 to 0.90 dl/g.
 4. The biaxiallyoriented polyester film of claim 1, wherein the endothermic peak (B) hasan endothermic energy of at least 0.5 mJ/mg.
 5. The biaxially orientedpolyester film of claim 1, which has a density of 1.345 to 1.370 g/cm³.6. The biaxially oriented polyester film of claim 1, wherein twodirections intersecting at right angles, in which the rate of increasein deformation after the film is placed under a load of 0.75 kg/mm² at80° C. for 30 hours is 0.070% or less and in which the amount ofresidual deformation after the load is removed is 0.090% or less, arepresent within the plane of the film.
 7. The biaxially orientedpolyester film of claim 6, wherein the rate of increase in deformationis 0.065% or less.
 8. The biaxially oriented polyester film of claim 6,wherein the amount of residual deformation is 0.085% or less.
 9. Thebiaxially oriented polyester film of claim 1, wherein two directionsintersecting at right angles, in which the 1% elongation stress is 4.5to 7.0 kg/mm², are present within the plane of the film.
 10. Thebiaxially oriented polyester film of claim 9, which has a 1% elongationstress of 4.7 to 6.8 kg/mm².
 11. The biaxially oriented polyester filmof claim 1, wherein two directions intersecting at right angles, inwhich the Young's modulus is 400 to 700 kg/mm , are present within theplane of the film.
 12. The biaxially oriented polyester film of claim 1,which has a refractive index in a thickness direction of 1.490 to 1.530.13. The biaxially oriented polyester film of claim 12, which has arefractive index in a thickness direction of 1.495 to 1.520.
 14. Thebiaxially oriented polyester film of claim 1, which has a moisture-vaportransmission of 0.03 to 0.35 g/m² ·mm·24 hr or less.
 15. The biaxiallyoriented polyester film of claim 1, wherein two directions intersectingat right angles, in which the thermal shrinkage factor after 30 minutesof a heat treatment at 150° C. is 0.20% or less, are present within theplane of the film.
 16. The biaxially oriented polyester film of claim 1,which has a plane orientation coefficient of 0.242 to 0.270.
 17. Thebiaxially oriented polyester film of claim 1, which has a center lineaverage surface roughness of 5 to 200 nm.
 18. The biaxially orientedpolyester film of claim 1, which has a thickness of 12 to 250 μm. 19.The biaxially oriented polyester film of claim 1, which has a thicknessnon-uniformity of 10% or less.
 20. A process for producing the biaxiallyoriented polyester film of claim 1, which comprises subjecting abiaxially oriented polyester film which has been produced from apolyester containing at least 80 mol % of ethylene-2,6-naphthalenedicarboxylate recurring units in accordance with a method known per se,to the following steps in the order named:(1) the step of heating thefilm under no strain at a temperature of 150 to 180° C. for 1 to 5hours; and (2) the step of heating the film in an unrolled or rolledstate at a temperature of 80 to 122° C. for 5 to 200 hours.
 21. Amembrane switch comprising the biaxially oriented polyester film ofclaim
 1. 22. The membrane switch of claim 21, which is embedded in theseat of a car to distinguish between an adult and a child by detectingthe weight of a seated person so as to adjust the inflation speed of anair bag which works at the time of a car crash.